In the past there have been air conditioning systems provided with a gas furnace unit and a heat pump unit, that perform air conditioning in a space to be air-conditioned. For example, Japanese Laid-open Patent Publication No. 64-54160 discloses an air conditioning system provided with a gas furnace unit and a heat pump unit, that switches between operating the gas furnace unit or the heat pump unit as a heat source unit at the moment a predetermined condition is fulfilled.